Most any liquid that contains particles that are not fully solubilized can be characterized as a colloidal suspension. Colloidal suspensions enjoy widespread use in applications ranging from advanced materials to drug discovery. Colloid based products include paints, inks, coatings, ceramic precursors, cosmetics, and pharmaceutical compositions. In the case of ceramics, concentrated colloidal suspensions may be fabricated into dense components by sintering.
The viscosity of colloid suspensions can vary over a wide range from free-flowing liquid to flocculated gel. Dispersants that modify viscosity are often added to colloidal suspensions. A major benefit of viscosity control is the ability to lower the viscosity of a concentrated suspension. By lowering the viscosity, a suspension may be processed through pumps, pipes, and other machinery in a simpler and more cost effective manner. By tailoring interactions between colloidal particles through the addition of a dispersant, one can alter the viscosity of colloids to make them suitable for use in a broad array of applications. Through dispersant addition, colloidal suspensions may be processed at higher solids content than would otherwise be possible.
Polyacrylic acid (PAA) is the polyelectrolyte dispersant most widely used for the aqueous processing of ceramics. PAA contains carboxylic acid groups, one per monomer unit, along its backbone. By adding PAA, the dispersion of the colloidal particles is increased, thus reducing aggregation or flocculation. As flocculation is reduced, viscosity decreases. Another common dispersant used in ceramics processing is poly(methacrylic acid) (PMAA). PMAA contains carboxylic acid groups and methyl substituents on the backbone.
These polyelectrolyte dispersants are believed to reduce flocculation by stabilizing the colloid particles through the negative charge generated when the carboxylic acid functional groups are deprotonated. When deprotonated or ionized, the negatively charged carboxylic groups are believed to form an electrostatic repulsive barrier between the particles that form the colloid, thus reducing flocculation. It has also been postulated that the steric requirements of the dispersant provides additional stabilization.
PAA dispersant systems become less effective at reducing viscosity when higher ionic strength colloidal suspensions, especially those containing multivalent ions, are involved. It is believed that the multivalent ions interfere with the electrostatic repulsive barrier of the carboxylic groups. This may result in a decreased electrostatic repulsive barrier between the colloidal particles, thereby increasing flocculation. Hence, there is a need for dispersants that can reduce flocculation of suspended particles and thus provide lowered viscosity in high ionic strength colloidal suspensions.